As a backup method of data in a volatile memory (main memory) in a controller of a conventional disk array device (hereinafter, simply referred to as a device (system)), there are a first method (FIG. 1A) and a second method (FIG. 1B) as described below.
(1) In the first method (“memory backup method”), when a blackout occurs, by using a battery, a power is supplied to a memory from the battery to retain data in the memory by a refresh operation or the like. In the first method, a time period (backup time) in which the data is retained with an operation of supplying the power to the memory (memory backup operation) is determined depending on a capacity (remaining amount) of the battery. A backup power (battery consumption amount) increases depending on the backup time, configuration of the device (memory capacity, etc.) and the like.
(2) In the second method (“nonvolatile memory evacuation method”), when a blackout occurs, data on the memory (main memory) is evacuated to a nonvolatile memory (backup memory).
A general blackout, particularly a short-time blackout will be supplementarily explained as follows. The short-time blackout includes “instantaneous voltage drop”, “momentarily blackout” and “instantaneous blackout”. “Instantaneous voltage drop” means that the voltage of a line power source drops instantaneously (for several milliseconds to several seconds). “Momentarily blackout” means that the line power source instantaneously turns off (for several milliseconds to several seconds). “Instantaneous blackout” means that the line power source instantaneously turns off (for several milliseconds to several seconds), which is generally longer than the momentarily blackout.
In addition, the short-time blackout generally occurs more frequently than the long-time blackout. FIG. 19 shows occurrences of the instantaneous voltage drop in Japan (cited from EMCC report No. 46-3). The horizontal axis represents a voltage drop ratio (“%”); the axis in a depth direction represents a continuous time (“cycle”); and the vertical axis represents a frequency (“times/year”). For example, an instantaneous voltage drop with 20% of the voltage drop ration and about 3-cycle of the continuous time most frequently occurs. As described above, the occurrence frequency such as the instantaneous voltage drop is relatively high. The blackout characteristics such as a total time of occurrences of blackout per year are different depending on a country or region.
Japanese Patent Application Laid-Open Publication No. 8-161236 (Patent document 1) discloses an example of the second method. That is when a voltage of a backup power source drops, data stored in a volatile memory is evacuated to a nonvolatile memory.